Photovoltaic cells are often used to recharge batteries, or to provide power to an electric grid and/or a building through an inverter. Photovoltaic cells often, however, provide less output power than expected from known device efficiency and illumination.
One reason that photovoltaic cells may deliver less than optimum power is that their maximum power output under typical conditions is often at a voltage that is not well matched to their load. This mismatch occurs, in part, because typical photovoltaic cells are temperature sensitive, and a sufficient quantity of photovoltaic cells must be connected in series to provide required voltage magnitude at high temperatures. This large photovoltaic cell count becomes excessive at low temperatures where photovoltaic cells' maximum power output voltage is highest. Similarly, maximum power output voltage may change with illumination changes. Other losses occur when any one series-connected photovoltaic cell in a module of interconnected photovoltaic cells (“photovoltaic module”) generates less current than other photovoltaic cells in the photovoltaic module. Barring additional circuitry, the output current of a series string of photovoltaic cells is effectively limited by photocurrent produced in the weakest, or most shaded, cell.
Since shading affects photocurrent produced in photovoltaic cells, often limiting current production of a series string of cells to that of a most-shaded cell of the string, un-shaded cells in the same series string may yield substantially less power than they are otherwise capable of. Further, shading of cells may vary with time of day, sun angle, obstruction position, and even the position of wind-blown leaves or other debris on a photovoltaic panel.
Maximum Power Point Tracking (MPPT) controllers are frequently connected between a photovoltaic module and a load, such as an inverter or a battery. MPPT controllers typically include a switching circuit, such as a buck DC-to-DC converter, that converts an input power at a module voltage to an output power for the load at a load voltage, and control circuitry that seeks to find a module voltage at which the photovoltaic module produces maximum power. The switching circuit of the MPPT controller serves to decouple the photovoltaic module and load voltages.
Parallel connection of photovoltaic modules in a photovoltaic system can result in reverse current flowing through portions of the system. For example, FIG. 1 illustrates a prior art photovoltaic electric power system 100 including three parallel-coupled strings 102, where each string 102 includes a plurality of MPPT controllers 104 with output ports 106 electrically coupled in series. A respective photovoltaic module 108 is electrically coupled to each controller 104's input port 110. In this document, specific instances of an item may be referred to by use of a numeral in parentheses (e.g., string 102(1)) while numerals without parentheses refer to any such item (e.g., strings 102).
Although the parallel-coupling of strings 102 in system 100 forces each string 102 to have a common voltage, strings 102 may have different inherent voltages. For example, strings 102 may receive unequal illumination, operate at different temperatures, and/or include differing numbers or types of photovoltaic modules 108 and associated MPPT controllers 104, causing strings 102 to operate at different inherent voltages. As another example, one string 102 may be intentionally disabled while another string 102 is enabled, such that the disabled string 102 has a lower inherent voltage than the enabled string 102. As yet another example, one string 102 may shut down more quickly than another string 102 during deactivation of system 100, or one string 102 may power up more quickly than another string 102 during activation of system 100, such that at least two strings 102 temporarily have different inherent voltages.
Mismatch in inherent electrical characteristics among strings 102 may lead to reverse current through some strings 102. For example, consider a scenario where string 102(1) has a higher maximum power point voltage than the open-circuit voltage of string 102(2) or 102(3), such that string 102(1) is considered a “strong” string and strings 102(2) and 102(3) are considered “weak” strings. The parallel-coupling of strong string 102(1) with weak strings 102(2) and 102(3) may cause weak strings 102(2) and 102(3) to operate in their negative current regimes, such that forward current 112 flows through strong string 102(1) and reverse currents 114, 116 flow through one or both of weak strings 102(2) and 102(3). If strings 102(2), 102(3) have different current-voltage characteristics, reverse currents 114, 116 will have different magnitudes, such that strings 102(2), 102(3) do not equally share reverse current. Such reverse current imbalance can be large due to the current-voltage characteristics of photovoltaic cells within photovoltaic modules 108, resulting in excessive power loss and system reliability issues. Accordingly, strings 102 sometimes include blocking diodes 118 to prevent flow of reverse current through the strings.
A photovoltaic module is typically capable of producing energy whenever it is exposed to light. This continuous availability of energy can present a safety hazard, such as during photovoltaic system installation, inspection, or maintenance. Additionally, it may be desirable to limit availability of energy from a photovoltaic module during a fault condition, or during an emergency situation, such as during a fire.